The present invention relates to a data communication system in which a central controller can simultaneously poll a plurality of remote stations and can resolve any contentions which occur when more than one remote station transmit messages in response to the poll request. More particularly, the present invention relates to a poll request which contains an address portion specifying the group of remote stations to be polled wherein the address portion is not required to have any more bits than are in the addresses identifying the remote stations.
Computer technologies involving data processing and data transmission have been adapted for use in building control and supervision because they offer advantages and economies which building control systems had not theretofore achieved. The prior art building control and supervision systems for maintaining comfort control and for monitoring fire and security points within the building were not able to easily minimize energy consumption in maintaining desired comfort levels nor minimize installation and hardware costs in wiring the central control function to the various loads being monitored and controlled. Instead of wiring the control function to each load separately and instead of requiring excessive amounts of hardware to achieve energy management functions, the modern building control system uses computer technology for connecting the various loads of a building over a single channel to the central control function and uses software and/or digital technology for achieving energy management control.
In a system comprising a central control station connected over a single communication channel to a plurality of remote stations, each station has a unique address and communicates with other stations by transmitting digital messages each having an address portion specifying the particular station to which the message is directed. The message will also have other portions, such as control and data portions, which may be utilized to require the addressed station to perform certain functions such as providing a status report, answer a polling message with any alarm messages which it may have to transmit to the central controller, requiring a remote station to perform a control function such as turning loads on or off, as well as requiring a wide variety of other functions.
The polling of remote stations in prior art systems was a time comsuming process. The central controller polled each of the remote stations one at a time in order to receive the desired alarm or status information. Because such a system was so time consuming, groups of stations were polled at one time. In such systems, if more than one remote station had information to report back to the central controller in response to the poll message, the reported messages clashed and the central controller received an indecipherable transmission. If such garbled messages were received, the central controllers were arranged to enter a contention poll routine in which the group to remote stations initially polled was subdivided into continuously smaller and/or different subgroups until the central controller received only clear messages.
These prior art contention polling systems have a number of drawbacks. Some systems were arranged so that each remote station was arranged to respond at a predetermined amount of delay after a poll message with each remote station having a different delay. Thus, contention may not occur until after several remote stations have transmitted. The central controller then prepared a list of stations which were possibly in contention starting with the first delay period at which contention occured and ending with the last station which might have transmitted, in view of transmission delays, once the controller realized that contention had taken place. The central controller was then arranged to poll each of the stations individually. This system required elaborate timing circuitry at each of the remote stations to generate the required delays and rquired complicated processing equipment at the central controller for not only recognizing which remote stations might be in contention but also for individually polling each of the possible contenders once it recognized that contention had occurred.
In other systems, the central controller simultaneously polled a group of stations and if more than one response was received and the responses conflicted with one another, the central controller conducted a poll of some lesser plurality or subgroup of the remote stations that were polled previously. The system kept subdividing until contending responses were eliminated. In these systems, however, the central processor transmitted both an address characterizing the first station of the poll and a second address characterizing either the extent of the poll or the address of the last station of the poll, thus requiring the transmission of two words to define the extent of the poll.
In systems of the nature described above, there has been the practice in some cases to transmit the information between the various stations of the system over the power lines of the building in which the system is located. Certain buildings such as large factories may require, however, more than one power feed into the building. It has been the practice in the prior art to connect the central controller to a master modem for communication with one of the power feeds and to interconnect the other power feeds by the use of repeaters. The use of a master modem with a plurality of repeaters depending on the number of power feeds makes isolation of the power feeds from one another more difficult and does not allow for the kind of flexibility which is required for systems of this type.